High-resolution sediment echosounding off Peru: Late Quaternary depositional sequences and sedimentary structures of a current-dominated shelf

About 6000 km of both bathymetric and high-resolution acoustic profiles were acquired on the shelf and upper slope offshore Peru between 9° S and 14° S. Two new sediment echosounder systems – SEL-96 and SES-2000DS – provided details of the sedimentary structures of the Quaternary sequences within the Sechura-Salaverry, Huacho and Pisco Basins. To a great extent, the poleward undercurrent determines the distribution of sediments. The undercurrent has generated numerous erosional unconformities, it has winnowed hardgrounds and has created mudwaves common between 250 m and 400 m water depth. Distinct subbottom reflectors within sedimentary units represent hiatuses due to periods of intensified winnowing or non-deposition. Erosional unconformities usually marked by pronounced reflectors suggest shifts of the undercurrent system related to climatic changes and eustatic variations of sea level. On a larger scale, the stacked prograding depositional sequences reflect the sea-level cycles of the Middle Pleistocene to the Holocene. Based on the stratigraphy of our piston cores and that of Ocean Drilling Program (ODP) Site 680, the depositional sequences limited by extended unconformities were assigned to oxygen isotope stages 1 to 7. Other sedimentary structures are small straight channels that were conduits for downslope sediment transport. Deformed sediments associated with synsedimentary normal faults result from creep movements indicating beginning slope failure.     

Stratigraphische Untersuchungen an Sedimentkernen des ibero-marokkanischen Kontinentalrandes

Zusammenfassung Biostratigraphie und Ergebnisse der Grobkornanalyse an Sedimenten vom marokkanischen und portugiesischen Kontinentalhang werden an zwei Kernen erläutert. Folgende Parameter scheinen sich besonders zur Korrelation zu eignen: Anteil des terrigenen Materiales an der Sandfraktion, Pteropodengehalt, Korngrößenverteilung des terrigenen Detritus, relative Temperaturangaben nach planktonischen Foraminiferen, Windungssinn von dreiGloborotalia- Arten. Beide Kerne erreichen Sedimente der letzten Kaltzeit, was dadurch unterstrichen wird, daß eistransportiertes, terrigenes Material und Hinweise auf Temperaturminima nach planktonischen Foraminiferen zusammenfallen.

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Biostratigraphy and the results of the sand grain analysis on Moroccan and Portuguese continental slope sediments are discussed. The investigation is based on samples taken from two cores. The following parameters seem to be well suited for correlation: The percentage of the terrigenous material of the sand-fraction, the pteropod content, grainsize distribution of the terrigeneous material, temperature curves based on planktonic foraminifera and coiling ratios of threeGloborotalia species.In both cores sediments from the last glacial period are reached. This is demonstrated by a very good correlation between the ice-transported terrigenous material and the temperature minima as determined by planktonic foraminifera.

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Résumé Notre présente étude concerne la biostratigraphie et l'analyse sédimentologique de deux carottes prélevées sur la pente continentale maroco-portugaise. Des corrélations ont été déduites de l'évolution des paramètres suivants: Pourcentage du matériel terrigène et des ptéropodes de la fraction sableuse, répartition granulométrique du matériel terrigène, variation faunique des foraminifères planctoniques, sens de l'enroulement de trois espèces deGloborotalia.Ces deux sondages pénètrent dans les sédiments de la dernière période glaciaire. La présence de matériel terrigène transporté par les glaces et les indications de température fournies par les foraminifères planctoniques s'harmonisent et confirment l'affirmation précédente.

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. : , Pterododen, , , Globorotalia. . .

     

Unreliable 14C dating of long-stored deep-sea sediments due to bacterial activity

Unreliability of radiocarbon ages obtained on the organic-carbon fraction of marine sediments stored wet and over a long period is explained as a consequence of bacterial up-take of recent CO₂. Deep-freezing, drying or immediate processing of samples intended for ¹⁴C age determination is recommended.     

Late Quaternary Seismic Stratigraphy of the Eastern Bengal Shelf

An ultra-high-resolution seismic study of the eastern Bengal Shelf with the parametric narrow-beam echosounder Parasound allows the interpretation of late Quaternary depositional patterns in terms of seismic stratigraphy. Accommodation space was still present on the outer shelf during the last lowstand, where a prograding delta developed in the western survey area. Oolitic beach ridges were later formed on top of this lowstand delta. Farther east, large parts of the shelf were exposed to subaerial erosion and a river system extended seaward across the area. A subaqueous highstand delta prograded southwards following the maximum transgression about 7,000 years ago. Its foreset beds exhibit acoustic voids very likely generated by sediment liquefaction, possibly caused by episodic energetic events such as major cyclones and/or earthquakes. Bottomset sediments extend seaward close to the shelf break in the west, whereas no Holocene sediments cover the outer shelf in the east.     

Seismostratigraphic analysis with centennial to decadal time resolution of the sediment sink in the Ganges-Brahmaputra subaqueous delta

The deltas of large rivers have become the focus of many research studies due to their vulnerability in times of expected sea level rise. One major delta in peril is the Ganges-Brahmaputra Delta (Bangladesh) due to its low elevation and high subsidence rate. In this study, high-resolution seismic and sediment echosounder data of the subaqueous part of the delta are analyzed by an integrated seismo-acoustic stratigraphic approach.Transparent Units (TUs) are the most prominent, continuously traceable architectural elements within the sigmoidal clinoform. TUs consist of homogenized sediments and are interpreted to be formed by liquefaction flows generated by subduction-related earthquakes of the nearby plate-boundary. The uppermost TUs are related to the historical well known major earthquakes which occurred in 1762, 1897 and 1950. Using the TUs as time marks the mean annual storage rates for the intervals could be assessed. A direct comparison of echosounder data gathered in 1994, 1997 and 2006 is used as another method to estimate the last decade mean annual storage rate.In general, the averaged decadal to centennial mean annual storage rates in the foreset beds have significantly decreased within the last ∼300 years from 22% to 18% and down to 13.8% of the present total annual fluviatile suspension supply to the delta (10⁹ t a⁻¹). This decrease, however, is not evenly distributed along the foreset beds. In the western clinoform mean annual storage rates slightly decreased during the last ∼300 years, whereas in the eastern clinoform the mean annual storage rate of the last decade is one-third of the initial value in the late eighteenth century.The variation of clinoform slope angles generally coincides with the local accumulation rates but a variable degree of asymmetry gives some new insights in the future trend of the subaqueous delta development. The sink of sediment in the western clinoform depocentre landward of the inflection point, where shape changes from convex to concave, is producing an increase in concavity.The decline in monsoon precipitation over the last centuries is assumed to have significantly contributed to the decreased mean annual storage rate in the eastern clinoform where the convexity of the clinoform increases seaward of the inflection point. It is very likely that the whole depocentre of the fluvial sediment load is shifting toward the western subaqueous delta and to the Swatch of No Ground Canyon and, thereby, increasing the export to the deep-sea fan.     

Improvement of deep-sea dredging operations through the use of a depth meter

A battery-operated depth meter mounted inside the dredge is used to monitor the time-depth profile of a dredge haul. By comparing is profile with the water depth recorded by the ship's echosounder, the locations at which dredge samples were collected can be determined. The analysis of the time-depth profile can also be used to estimate the minimum length of the dredge wire, thus increasing the efficiency of dredging operations.     

Experimental study of nearshore transportation of pebbles with attached algae

Direct observation of the transport behaviour of pebbles with attached algae, purposely laid out in the nearshore zone, showed that they can be easily transported by oscillatory water movements, if the ratio weight of pebble)/(wet weight of algae) is less than or equal to about 3.     

Hemipelagic Sediment Accumulation Rates in the South China Sea Related to Late Quaternary Sea-Level Changes

Sediments of 13 piston cores from opposite continental slopes of the South China Sea, off southern China and Sabah (northern Borneo), were analyzed by sedimentological methods and dated by oxygen isotope stratigraphy. Sediments mostly consist of hemipelagic clay with 20% carbonate off Sabah and 40% off China. We calculated terrigenous and carbonate accumulation rates for up to 11 time-slices from the Holocene to oxygen-isotope stage 6. Terrigenous accumulation rates generally increase with water depth and reach a maximum at the middle slope off Sabah and at the lower continental slope off China. During glacial and interglacial times this distribution pattern did not markedly change, despite an increase of accumulation rates for glacial periods by a factor of 2 to 5 compared to interglacial periods. Rates are negatively correlated with positions of sea level, which controls the partition of fluviatile terrigenous material for deposition on shelf, slope, and abyssal plain. Carbonate accumulation rates are higher off China by a factor of 2 compared to Sabah, probably indicating higher calcareous plankton productivity.     

Dampier Ridge,Tasman Sea,as a stranded continental fragment∗

Dredging of the Dampier Ridge recovered small fragments of granite, gabbro and sandstone. Dating of the igneous samples by the U‐Pb, K‐Ar and Rb‐Sr methods yielded precise ages mainly in the range 250 to 270 Ma, mid‐Permian. An imprecise Sm‐Nd mineral age of ～ 310 Ma might reflect slightly earlier emplacement of the gabbro. A granitic fragment has a composition approximating that of a minimum melt. Taken together with the Late Palaeozoic emplacement ages and other geochemical and geophysical data, the evidence strongly supports the conclusion that the Dampier Ridge is a continental fragment, formerly part of eastern Australia, with its present location a consequence of continental rifting and opening of the Tasman Basin by sea‐floor spreading.     

Stepwise,earthquake-driven coastal subsidence in the Ganges–Brahmaputra Delta (Sundarbans) since the eighth century deduced from submerged in situ kiln and mangrove remnants

Natural Hazards - This study reconstructs the coastal subsidence over the past 1300 years in a mangrove region along the coast of the Ganges–Brahmaputra Delta, an area not affected...